DEVELOPMENT OF ASSOCIATION AND CLIMAX CONCEPTS 335 



continuous transect method, opportunity to study the relationship of com- 

 munities, and hence of community types (Braun, 1935a, 1940, 1942). Grada- 

 tions related to altitude and to moisture (suggested by topographic position, 

 insolation, etc.) as well as to substratum occurred. The whole displayed a 

 multitude of segregates. The most mesic communities are the most highly 

 mixed communities, including in the canopy a great number of tree species, 

 some with wide ecological amplitude, which are species with many biotypes, 

 and hence a variety of ecotypes, as Dr. Whittaker ably explained in a recent 

 paper on the vegetation of the Great Smoky Mountains (1956). Partly be- 

 cause these very mesic mixed communities often contain a few red oak, white 

 oak, and chestnut trees. Dr. Whittaker has compared them with transition 

 forests of the Smokies, where Castanea is classed as "subxeric." However, 

 in the Cumberland Mountains and on the Cumberland Plateau, Castanea has 

 a greater variety of ecotypes and may be associated with river birch and 

 sweet gum on river bottoms, with sweet gum, pin oak, and beech on swamp 

 flats; that is, it varies from hydromesic to subxeric, but reaches greatest 

 abundance in submesic and subxeric sites, where it may be codominant with 

 sugar maple and tuliptree in the former, or with chestnut oak in the latter. 

 Other species, also, have wider ecological amplitudes in the Cumberlands than 

 in the Smokies. 



To return briefly to concepts — all the mesic mixed communities have much 

 in common, in species and in luxuriance and coverage of herbaceous vegeta- 

 tion. In a scheme of classification, all could be grouped under the broad term 

 mixed mesophytic, that is, as belonging to the Mixed Mesophytic association. 

 But because no two species are alike in physiological requirements, gradation 

 in environment results in segregation of the mixture. For these interrelated 

 similar communities the term association-segregate was proposed as a dy- 

 namic term suggesting response to environmental forces at work (Braun, 

 1935). It was apparent that gradations led from the mesic communities readily 

 classified as belonging to the mixed mesophytic type to less mesic transitional 

 types and finally to subxeric types on ridges. It was also apparent that a more 

 or less distinct break could be found where influence of canopy (late leafing 

 of oak and chestnut and slow decomposition of their leaf litter) was reflected 

 in composition of undergrowth. This break occurs between what may be 

 classed as mixed mesophytic communities and oak-chestnut communities. 



In the Cumberland Mountains, mixed mesophytic communities occupy not 

 only the coves but much of the slopes, except convexities and ridges. Nowhere 

 else are such communities better developed. This I believe is because of (1) 

 the long period of continued occupancy of an area of mature topography; (2) 

 the very deep soils which can develop where underlying strata are horizontal 

 or nearly so (in marked contrast to conditions in the Great Smoky Mountains 

 and Blue Ridge) ; (3) the high annual precipitation, equably distributed and 

 without the occurrence of summer droughts, such as the Southern Appalachians 



